The present invention relates to a television video signal scrambling system and, more particularly, to a video signal scrambling system for use in a wireless pay television broadcasting system in which a broadcasting station transmits television signals scrambled by inverting video signals by an encoder for providing sustaining programs at charge so that only specified subscribers reproduce the television signals on their television receivers by decoding the scrambled signals with their decoders.
The charged wireless television system generally referred to as a pay television system was developed in reply to a demand for more substantial and enriched programs even with charge in view of dull commercial messages indispensable to charge-free commercial broadcasting programs and deterioration in the quality of the programs due to various restrictions. The future of the pay television system is noteworthy because it has significant advantages such that cost and time for laying cable can be saved and that a system can theoretically accomodate a limitless number of subscribers because it is a wireless system.
In a wireless pay television system intended for specified subscribers only, television signals and audio signals are generally scrambled for foreclosing non-subscibers from receiving the program. A conventional video signal scrambling system is shown in FIG. 1, in which a video signal applied to an input terminal 1 is inverted in its polarity by a video signal polarity inverter 3 and applied thereby to an output terminal 2. The video polarity inverter 3 is controlled by a random number signal generator 4, that is, the video polarity inverter 3 provides the video signal with its polarity inverted or uninverted corresponding to a randomly changing logic value signal from the random number signal generator 4. A decoder can decode and restore the scrambled video signals according to the prearranged key code signal. Thus, the conventional wireless pay television system is so arranged that only the specified subscribers having the decoders, respectively, can satisfactorily receive the programs.
The conventional video signal scrambling system has a disadvantage that, when a subscriber restores with his decoder video signals transmitted by the broadcasting station in a form inverted by the encoder, if there is a lag between the original video signals and the restored video signals caused by the difference in characteristics (for example, linearity) between the component parts (such as diodes, transistors and the like) used in the encoder and in the decoder, flickering is caused in the picture due to the irregular brightness and the variation in the hue and the like. In order to avoid this disadvantage, it is necessary to equalize the characteristics of the component parts of the encoder and the decoders and to regulate and maintain them strictly. This, however, is not preferable in view of the difficulties in manufacture and economy.